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Monosubstituted [M(N)Cl(2)(POP)] [M = Tc, 1; Re, 2] and [M(N)Cl(2)(PNP)] [M = Tc, 3; Re, 4] complexes were prepared by reaction of the precursors [M(N)Cl(4)](-) and [M(N)Cl(2)(PPh(3))(2)] (M = Tc, Re) with the diphosphine ligands bis(2-diphenylphosphinoethyl)ether (POP) and bis(2-diphenylphosphinoethyl)methoxyethylamine (PNP) in refluxing dichloromethane/methanol solutions. In these compounds, the diphosphine acted as a chelating ligand bound to the metal center through the two phosphorus atoms. Considering also the weak interaction of the heteroatom (N or O) located in the middle of the carbon backbone connecting the two P atoms, we found that the coordination arrangement of the diphosphine ligand could be viewed as either meridional (m) or facial (f), and the resulting geometry as pseudooctahedral. The heteroatom of the diphosphine ligand was invariably located trans to the nitrido linkage, as established by X-ray diffraction analysis of the representative compounds 2m and 4f. Density functional theoretical calculations showed that in POP-type complexes the mer form is favored by approximately 6 kcal mol(-1), whereas mer and fac isomers are almost isoenergetic in PNP-type complexes. A possible role of noncovalent interactions between the phosphinic phenyl substituents in stabilizing the fac-isomer was also highlighted. The existence of fac-mer isomerism in this class of complexes was attributed to the strong tendency of the two phosphorus atoms to occupy a reciprocal trans-position within the pseudooctahedral geometry. The switching of P atoms between cis- and trans-configurations was confirmed by the observation that the fac isomers, 1f and 2f, were irreversibly transformed, in solution, into the corresponding mer isomers, 1m and 2m, thus suggesting that fac complexes are more reactive species. Theoretical calculations supported this view by showing that the lowest unoccupied orbitals of the fac isomers are more accessible to a nucleophilic attack with respect to those of the mer ones. Furthermore, the large participation of the Cl orbitals to the HOMO, which is a metal-ligand pi* antibonding in the complex basal plane, shows that the Tc-Cl bonds are labile. As a consequence, facial isomers could be considered as highly electrophilic intermediates that were selectively reactive toward substitution by electron-rich donor ligands. Experimental evidence was in close agreement with this description. It was found that fac-[M(N)Cl(2)(PXP)] complexes easily underwent ligand-exchange reactions with bidentate donor ligands such as mercaptoacetic acid (NaHL(1)), S-methyl 2-methyldithiocarbazate (H(2)L(2)), diethyldithiocarbamate sodium salt (NaL(3)), and N-acetyl-L-cysteine (H(2)L(4)) to afford stable asymmetrical heterocomplexes of the type fac-[M(N)(L(n))(POP)](+/0) (5-8) and fac-[M(N)(L(n))(PNP)](+/0) (9-14) comprising two different polydentate chelating ligands bound to the same metal center. In these reactions, the bidentate ligand replaced the two chloride atoms on the equatorial plane of the distorted octahedron,...

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The potentially bidentate hybrid ligand (o-hydroxyphenyl)diphenylphosphine, abbreviated POH, reacted via ligand-exchange with pentavalent rhenium precursors to give a series of six-coordinate mono- and dioxo complexes. Accurate control of the metal:ligand stoichiometric ratio allowed for the isolation of the mono-substituted [ReOCl3(PO)]- (1) and [ReOCl2(PO)(PPh3)] (2) derivatives. 1 was found to be the key intermediate for the syntheses of three more types of bis-substituted compounds:  anionic dioxo [ReO2(PO)2][A] (A = NBu4 (3), AsPh4 (4)), neutral monooxo [ReOX(PO)2] (X = Cl (5), Br (6), I (7)), and neutral monooxo mixed-ligand [ReOX(PO)(PNH)] [PNH = (o-amidophenyl)diphenylphosphine; X = Cl (8), Br (9), I (10)] complexes. In the mono-substituted complexes, the P,O-donors of the bidentate ligand spanned an equatorial (P) and the apical position (O) trans to the ReO linkage in a distorted octahedral arrangement. In all of the bis-substituted monooxo compounds, the second chelate ligated on the equatorial plane almost orthogonally positioned with respect to the first one, the two phosphorus donors showing a mutual cis-(P,P) orientation. Dioxo complexes retained the cis-(P,P) configuration with the bidentate ligands symmetrically coordinated on the equatorial plane normal to the trans-ReO2 core. All the complexes were characterized by various physical techniques, including IR, MS, and 1H/31P{1H} NMR. The X-ray structure of a representative compound for each category, namely [ReOCl3(PO)][NBu4] (1), [ReO2(PO)2][AsPh4] (4), [ReOCl(PO)2] (5), and [ReOCl(PO)(PNH)] (8), were determined. Crystals of 1 were monoclinic, P21/n, a = 10.840(3) Å, b = 22.167(6) Å, c = 15.210(4) Å, β = 95.91(2)°, and Z = 4; those of 4 were triclinic, P1̄, a = 12.679(7) Å, b = 13.082(7) Å, c = 19.649(8) Å, α = 82.64(4)°, β = 81.16(4)°, γ = 62.27(3)°, and Z = 2; those of 5 were orthorhombic, a = 10.225(4) Å, b = 14.208(6) Å, c = 21.771(9) Å, P212121, and Z = 4; and those of 8 were orthorhombic, a = 10.199(2) Å, b = 14.147(4) Å, c = 21.772(6) Å, P212121, and Z = 4. The four structures were solved by the Patterson method and refined by full-matrix least-squares procedures to R = 0.050, 0.063, 0.043, and 0.039 for 1, 4, 5 and 8, respectively. Both solution state (31P{1H} NMR) and solid state (X-ray) demonstrated a cis-(P,P) arrangement for each bis-substituted complex, with the Re atom at the center of a highly distorted octahedron. Detailed analyses of the IR spectra of this series of Re(V) compounds in the region 900−580 cm-1 allowed us the possibility to distinguish between symmetrical and asymmetrical bis-substituted complexes.